Noise eliminator for radio receiver

ABSTRACT

A noise eliminator for a radio receiver comprises an automatic gain control amplifier for amplifying a noise signal extracted from an intermediate-frequency signal or a detector output signal of the receiver, a circuit for producing a faithfully pulsating noise signal and an average noise signal from the output of the amplifier circuit, an operational circuit for detecting an impulse noise component through a comparative operation of the pulsating noise signal and the average noise signal, and a gate circuit for controlling a signal transmission in the receiver in accordance with the detected impulse noise component.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates principally to a noise eliminator for aradio receiver, and more particularly to an improvement capable ofachieving effective elimination of an impulse noise such as an ignitionnoise from an internal combustion engine.

2. Description of the Prior Art

In advance of an explanation on the noise eliminator of the presentinvention, a conventional device of this kind will be described briefly.

FIG. 1 shows a block diagram of a known noise eliminator which comprisesa noise detection circuit 3 for detecting an impulse noise signal whicharrives from an FM detector 2 in an FM receiver, a control-pulse formingcircuit 4 for producing a control pulse signal out of the noise signaldetected by the circuit 3, and gate circuits 5 for controlling thesignal paths at outputs of a multiplex demodulator 12 by the controlpulse signal obtained from the circuit 4. In this network, an FM noiseis detected by introducing the FM detection output into a 100 KHzhigh-pass filter 6 and a rectifier 7. The noise signal thus detected isamplified by an amplifier 8 and then is fed to a monostablemultivibrator 9 in the control-pulse forming circuit 4 so as to triggerthe multivibrator 9. After being integrated by a unidirectionalintegrator 10, the output of the multivibrator 9 is shaped into adesired waveform by a waveform shaper, e.g. a Schmitt trigger circuit 11to become a control pulse signal, which is then fed to the gate circuits5. In response to arrival of the control pulse signal, the gate circuits5 serve to interrupt the output signal paths of the multiplex stereodemodulator 12 to prevent the noise from appearing in the demodulatedoutput signals.

In such a noise eliminator as described above, however, there existsdifficulty in properly setting the absolute triggering level for themonostable circuit 9 due to the variety of the noise component levelwhich varies in accordance with the antenna input level. If thetriggering level is relatively low, the gate circuits are frequentlyrendered nonconductive and the transmission of the demodulator outputsignals is unnecessarily interrupted resulting in poor efficiency of thesignal transmission. On the contrary, when the triggering level isrelatively high, the impulse noise will not be eliminated effectively.

SUMMARY OF THE INVENTION

In view of the above-mentioned circumstances regarding the conventionalnoise eliminator, the present invention has been accomplished in anattempt to provide an improved circuit which is capable of achievingeffective elimination of impulse noises in a radio receiver with asimplified circuit configuration and a reduced production cost. Thefeatures of the invention reside in the provision of a filter circuitfor extracting a noise signal, a circuit for producing a pulsating noisesignal and an average noise signal from the filter output, anoperational circuit for detecting an impulse noise component through acomparative operation of the pulsating and average noise signals, and agate circuit for controlling a signal transmission in the receiver inaccordance with the detected impulse noise component obtained from theoperational circuit.

In a preferred embodiment of this invention, a hold circuit is providedat the output side of the gate circuit to hold an audio signal duringthe conductive state of the gate circuit.

In the noise eliminator of the invention, any impulsive noise componentabove the average noise level or a predetermined threshold level isextracted for controlling the receiver signal path, so that theimpulsive noise is always eliminatable with accuracy regardless of theantenna input level.

Accordingly, it is an object of the present invention to provide animproved noise eliminator for use in a radio receiver capable ofeffectively eliminating impulse noise signals regardless of the level ofan antenna input signal.

Another object of the invention resides in providing a noise eliminatorwhich is simple in circuit configuration and low in manufacturing cost.

Further objects of the invention and advantageous features thereof willbe apparent to those skilled in the art from the following descriptionof a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter the present invention will be described in detail withreference to the accompanying drawings illustrating an embodimentthereof.

FIG. 1 is a block diagram showing a typical example of a conventionalnoise eliminator;

FIG. 2 is a circuit diagram of an example of a noise eliminatoraccording to the invention applied to an FM radio receiver; and

FIG. 3 (a), (b), (c) and (d) illustrate waveforms useful to explain theoperation of the circuit in FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 2, an input signal received at an antenna 21 ishigh-frequency amplified and is further converted into anintermediate-frequency (IF) signal in a front end 22, which issubsequently amplified in an IF amplifier 23. The signal therefrom is FMdetected in an FM detector 24, whose output is then fed to a multiplexstereo demodulator 25 where demodulated outputs of left and rightchannels are provided. In the meanwhile, the FM detection outputobtained at the output terminal of the FM detector 24 is fed via ahigh-pass filter 27 to the base of a transistor Q1 in an automatic gaincontrol (AGC) amplifier circuit 28. The high-pass filter 27 has, forexample, a critical frequency of 100 KHz.

In the amplifier circuit 28, the signal received from the high-passfilter 27 is amplified by transistors Q1 and Q2, whose output isrectified by a diode D1 shunting the positive portions of the output andthen is smoothed by a resistor 29 and a capacitor 30 to produce asmoothed output, which is fed back via a resistor 31 to the base of thetransistor Q1. Thus, the rectified output of the diode D1 appearing at ajunction point P1 is subjected to automatic gain control in which thegain of the amplifier circuit 28 is controlled to be small when theoutput of the filter 27 increases. In this circuit configuration, acurrent meter 33 is connected to an emitter resistor 32 located on theemitter side of the transistor Q1 so that the meter 33 indicates thenoise intensity which is inversely proportional to an emitter current ofthe transistor Q1. The junction point P1 is connected to an inputterminal 38 of an operational circuit 37 via an integrator 36 consistingof a resistor 34 and a capacitor 35, and is also connected to anotherinput terminal 42 of the operational circuit 37 via an integrator 41consisting of a resistor 39 and a capacitor 40. The integrator 41 has atime constant much greater than that of the integrator 36 so that theintegrator 41 produces an average signal of the noise signal appearingat point P1 while the integrator 36 produces a pulsating signal which ismore faithful in wave shape to the above-said noise signal than saidaverage signal is.

The operational circuit 37 executes, at a predetermined threshold level,comparative operation of the signals obtained at the input terminals 38and 42, wherein the terminal 38 is connected to the base of a transistorQ3 via a reversely connected diode D2 serving to set the thresholdlevel, while the terminal 42 is connected to the base of a transistorQ4.

The transistors Q3 and Q4 constitute a differential amplifier, and aresistor 43 is connected between the respective bases. A common emitterthereof is connected to a positive power supply +B via aconstant-current source 44, and the collector of the transistor Q3 isconnected to a negative power supply -B, while the collector of thetransistor Q4 is connected to the base of a transistor Q5 in the nextstage and also to the negative power supply -B via a load resistor 45.The collector of the transistor Q5 is connected to the base of atransistor Q6 while being grounded via a resistor 46, and the emitter ofthe transistor Q5 is connected to the negative power supply -B. As forthe transistor Q6, the collector thereof is connected to input terminals48a and 48b of control circuits 47a and 47b while being grounded via aresistor 49, and the emitter thereof is connected to the negative powersupply -B. The two transistors Q5 and Q6 constitute a circuit to producea negative control pulse signal from the output of the operationalcircuit 37. Each of the control circuits 47a and 47b consists of afield-effect transistor Q7 which is connected in series with the outputsignal path of the demodulator 25, and a capacitor 51 inserted at theoutput side of the field effect transistor between the signal path andthe ground, wherein the control pulse signal is applied to the gate ofthe field-effect transistor Q7 via a resistor 52. The respective controlcircuits 47a and 47b deliver output signals to output terminals 26a and26b of the receiver.

In the network of the above configuration, impulse noise elimination isperformed as follows. First the FM detection output of the FM detector24 is introduced to the high-pass filter 27, from the noise component isselectively obtained and then is fed to the AGC amplifier 28, where thenoise component is amplified with automatic gain control. In this case,the meter 33 is driven by the emitter current of the transistor Q1varying in accordance with the noise component. The noise signalamplified and rectified by the AGC amplifier circuit 28 is obtained atthe junction point P1 and is fed to the integrators 36 and 41. Theintegrator 36 produces a pulsating noise signal as shown in FIG. 3 (a)from the noise signal at the junction point P1 and feeds the same to oneinput terminal 38 of the operational circuit 37. In the meanwhile, theintegrator 41 produces an average noise signal shown in FIG. 3 (b) fromthe noise signal at the junction point P1 and feeds the same to theother input terminal 42 of the operational circuit 37. Accordingly,voltage signals VA and VB appear across the resistor 43 wherein thesignal VA is the pulsating noise signal beyond the threshold level Vt,i.e. about 0.6 volt in absolute value determined by the diode D2 and thesignal VB is the average noise signal. In other words, at the output ofthe differential amplifier is obtained a difference signal between thepulsating noise signal and the average noise signal and beyond thethreshold level as shown in FIG. 3 (c), so that an impulse noise signalis effectively extracted through the comparative operation between therelatively faithful noise signal and the average noise signal. Theimpulse noise signal thus extracted is a noise signal beyond the averagenoise signal level and the threshold level.

It is to be noted here that the integrator 36 may be spared, though itacts to make the phase of the pulsating noise signal exactly coincidewith that of the average noise signal obtained through the integrator41. The diode D2 may also be spared but is preferably provided to setthe threshold level so that the noise eliminator does not respond tosmall negligible impulse noises.

The extracted impulse noise signal is amplified by the transistors Q5and Q6, and the control pulse signal shown in FIG. 3 (d) obtained fromthe collector of the transistor Q6 is applied to the gate of thefield-effect transistor Q7 in the control circuits 47a and 47b. Inresponse to arrival of the control pulse signal, each field-effecttransistor Q7 is rendered nonconductive to interrupt the output signalpath 50 of the demodulator 25 thus preventing an impulse noise frombeing reproduced. This signal interruption does not cause any seriousproblem to the reproduced sound since the signal having been held in thecapacitor 51 is supplied to the receiver output during suchinterruption.

The input to the high-pass filter 27 in the above embodiment may be anintermediate-frequency signal obtained from the IF amplifier 23, insteadof the FM detection output fed via the high-pass filter 27. This is trueespecially when the invention is applied to an AM radio receiver.

What is claimed is:
 1. A noise eliminator for a radio receiver,comprising:first circuit means coupled to a signal path leading from afront end circuit to a demodulator circuit in the receiver and includinga filter circuit for deriving a noise signal contained in a signal onsaid signal path; second circuit means receiving said noise signal foraveraging the same to produce an average signal; third circuit means forproducing a differential signal corresponding to a difference betweensaid noise signal and said average signal; gating means inserted betweensaid demodulator circuit and an output terminal of the receiver forinterrupting a transmission of a demodulated output signal to saidoutput terminal when said gating means is nonconductive; and fourthcircuit means responsive to said differential signal for rendering saidgating means nonconductive.
 2. The noise eliminator according to claim1, in which said third circuit means includes threshold means forsetting a threshold level so that said third circuit means delivers outsaid differential signal which exceeds said threshold level.
 3. Thenoise eliminator according to claim 1, in which said second circuitmeans comprises a first integrating circuit and a second integratingcircuit having a greater time constant than that of the firstintegrating circuit so that said second integrating circuit producessaid average signal and said first integrating circuit produces apulsating signal which is more faithful in wave shape to said noisesignal than said average signal is.
 4. The noise eliminator according toclaim 3, in which said third circuit means comprises a differentialamplifier having a pair of inputs connected respectively to outputs ofsaid first and second integrating circuits, and a resistance elementinterconnecting said pair of inputs of the differential amplifier. 5.The noise eliminator according to claim 4, in which said third circuitmeans further comprises threshold means connected between the output ofeither of said first and second integrating circuits and thecorresponding input of said differential amplifier for establishing athreshold level so that the third circuit means produces saiddifferential signal exceeding the threshold level.
 6. The noiseeliminator according to claim 5, in which said threshold means comprisesa diode connected between the output of said first integrating circuitand the input of said differential amplifier.
 7. The noise eliminatoraccording to one of claims 1 through 6, in which said first circuitmeans further comprises a gain-controlled amplifier connected to anoutput of said filter circuit, and gain controlling means forcontrolling the gain-controlled amplifier in accordance with an outputsignal of said gain-controlled amplifier.
 8. The noise eliminatoraccording to claim 7, in which said gain controlling means comprises anegative feedback circuit including a rectifier connected to an outputof said gain-controlled amplifier and a feedback path leading from saidoutput to an input both of said gain-controlled amplifier.
 9. The noiseeliminator according to claim 4, in which said fourth circuit meanscomprising an amplifier for amplifying an output signal of saiddifferential amplifier to produce a control signal suited for actuatingsaid gating means.
 10. The noise eliminator according to one of claims 1through 6, further comprising a hold circuit connected parallel to asignal path leading from said gating means to said receiver outputterminal.
 11. The noise eliminator according to claim 1, in which saidfilter circuit comprises a high-pass filter having a critical frequencyof 100 KHz.
 12. The noise eliminator according to claim 1, in which saidreceiver is an FM receiver and said demodulator is a multiplexdemodulator, the receiver comprising an intermediate frequency amplifierconnected to an output of said front end circuit and an FM detectorconnected to an output of said intermediate amplifier and followed bysaid multiplex demodulator, and in which said first circuit means isconnected to an output of said FM detector and said filter circuit is ahigh-pass filter.